This application claims Paris Convention priority of DE 101 17 595.7 filed Apr. 7, 2001 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a main field magnet arrangement in one-sided magnetic resonance devices for generating a homogeneous magnetic field B in a working volume, wherein the working volume and the main field magnet are disposed on different sides of a plane E and the main field magnet has a geometric shape which permits arrangement of a radio frequency (RF) receiver coil system and optionally a gradient coil system on the same side of the plane E as the main field magnet wherein field-generating elements of permanent-magnetic material are provided which are magnetized in the same sense and in the direction of the magnetic field B in the working volume and which have a radially outer part and a radially inner part relative to an axis A extending through the center of the working volume, wherein the surface, facing the working volume, of the radially inner part is axially spaced further apart from the center of the working volume than the surface, facing the working volume, of the radially outer part, with a depression being formed in the axial direction in the radially inner part.
An arrangement of this type is e.g. known from U.S. Pat. No. 5,959,454.
An appropriate mathematical description of the spatial dependence of the magnetic induction B(xcfx81,xcex8,xcfx86) as a function of the spherical coordinates xcfx81,xcex8,xcfx86 (xcfx81: radius, xcex8: polar angle relative to the axis, xcfx86: azimuthal angle) about a point in the center of the working volume is given by the expansion according to spherical harmonic functions:       B    ⁡          (              ρ        ,                  xe2x80x83                ⁢        θ        ,                  xe2x80x83                ⁢        φ            )        =            ∑              l        =        0            ∞        ⁢          xe2x80x83        ⁢                  ∑                  m          =                      -            1                                    +          1                    ⁢              xe2x80x83            ⁢                        C                      l            ,            m                          ⁢                  xe2x80x83                ⁢                  ρ          1                ⁢                              P                          l              ,              m                                ⁡                      (                          cos              ⁢                              xe2x80x83                            ⁢              θ                        )                          ⁢                  xe2x80x83                ⁢                  (                                    cos              ⁢                              xe2x80x83                            ⁢                              m                ⁢                φ                                      +                          i              ⁢                              xe2x80x83                            ⁢              sin              ⁢                              xe2x80x83                            ⁢                              m                ⁢                φ                                              )                    
Pl,m (cos xcex8): associated Legendre polynomial of degree l and order m,
Cl,m: Amplitude, i2=xe2x88x921
The amplitudes Cl,m are complex numbers. Since the magnetic induction B is real, the amplitudes Cl,m with identical l and oppositely equal m are complex conjugates of each other. For magnets which should generate as homogeneous a magnetic field in a working volume as possible, the term with l=m=0 is the only desired term since it describes an ideally homogeneous magnetic field. All other terms are disturbance terms which impair homogeneity. In the geometrical design of main field magnets for generating homogeneous magnetic fields, one therefore tries to eliminate as many disturbance terms of a degree l greater than 0 as possible up to as large a value l0 as possible through optimization of the geometric arrangement of the main field magnet. In these cases, the dependence of magnetic induction B along any straight line through the center of the working volume is given by superposition of the desired constant term with parabolas of order  greater than l0+1. The latter provide only negligible contributions to the magnetic field proximate the origin which is therefore homogeneous through a limited working volume, and is suitable for magnetic resonance method examinations. For rotationally symmetric magnet arrangements, the magnetic field is also rotationally symmetric such that for arrangements of this type all terms with m unequal 0 theoretically disappear. The second summation completely vanishes and the number of possible disturbance terms of amplitudes Cl,m to be compensated when designing the arrangement, is considerably reduced. For magnet arrangements which are additionally mirror-symmetrical in the axial direction relative to the center of the working volume, all disturbance terms disappear with even-numbered degree l. In these cases, the geometric design of the main field magnet must eliminate only very few, e.g. five, disturbance terms to obtain a theoretically very large working volume. For these reasons, commercial main field magnets for magnetic resonance devices according to current prior art usually have a mirror symmetry in the axial direction relative to the center of the working volume and are always rotationally symmetric. Deviations from pure rotational symmetry exist, if at all, due to magnetic components of the magnet arrangement which are far away from the respective working volume, such as magnetic yokes of iron in magnet arrangements having iron shielding. The components of magnet arrangements which are close to the working volume are in any case rotationally symmetric. In practice, none of the magnet arrangements mentioned herein is suited for generating sufficiently homogeneous magnetic fields on its own since unavoidable tolerances during production generate relatively small disturbance terms of a low degree l and low order l which are, however, still inadmissibly large for the application and which therefore must be compensated for with a suitable additional magnet arrangement, the shim system, which can be precisely set.
U.S. Pat. No. 4,689,591 describes arrangements of magnetic field generators, mainly magnet coils which are generally rotationally symmetric relative to an axis, however, not symmetrical in the direction of the axis. In this reference, permanent magnets are also used as magnetic field generators which are preferably rod-shaped and are magnetized in the direction of their longitudinal axes. A concrete design of such magnet arrangements is not disclosed.
U.S. Pat. No. 5,959,454 discusses a class of main field magnets which consists exclusively or principally of permanent-magnetic components which are exclusively rotationally-symmetrical about a common axis. The magnet arrangements according to U.S. Pat. No. 5,959,454 comprise a radially outer ring of permanent magnetic material which is magnetized in the direction of the axis. A rotationally-symmetrical insert of permanent-magnetic material, which is also magnetized in the same direction as the outer ring, is disposed inside the outer ring. In the most simple form, the insert also comprises one or more nested rings and possibly an full inner cylinder. The particularly advantageous property of these arrangements consists in that the insert represents a depression on the side of the magnet arrangement facing the working volume which can be sufficiently large that a relatively large working volume with sufficiently homogeneous and relatively strong magnetic field becomes possible.
Moreover, there is sufficient space in the depression for receiving an actively shielded one-sided gradient coil system and an RF coil. The magnetic field in the working volume of these arrangements has an axial orientation (z direction). The magnetic induction in the working volume in the embodiment of U.S. Pat. No. 5,959,454 is approximately 14% of the magnetic remanence polarization of the magnetic material. With the magnetic material NdFeB having a remanence polarization of 1.3 T, one obtains an induction of 0.18 T which is within the usual range of commercial magnetic resonance devices.
The magnet arrangements according to U.S. Pat. No. 5,959,454 are rotationally symmetrical but are designed for applications where mirror-symmetry relative to the center of the working volume is impossible. In the embodiments disclosed therein, all disturbance terms of the order m=0 disappear due to the rotational symmetry and all disturbance terms of the degrees 1, 2, 3, 4 disappear due to the further geometrical design. However, disadvantageously, the exclusively permanent-magnetic components must be rotationally symmetric. NdFeB, which is preferably used for such magnets, is not suited for rotationally-symmetric shaping since it cannot be processed by turning on the lathe. Grinding into shape is the preferred processing method for this material. Although grinding into a cylindrical shape is possible, flat surfaces are nevertheless preferable. A further disadvantage consists in that the shim system must be accommodated in the depression thereby reducing the space for the gradient coil system and the RF coil. Although only little magnetic material is needed for the shim system, a relatively large amount of space is nevertheless required for the mechanical holding device of the magnetic material for the shim system. Disposal of the shim system behind the magnet arrangement, (see U.S. Pat. No. 5,959,454) is not suitable in practice, since the large separation causes the magnetic fields in the working volume which are produced by the shim system to be too weak. Although realization of the overall system of main field magnets, gradient coil system, shim system and RF coil with the shim system accommodated in the depression of the main field magnet is possible, more space in the depression would be desirable.
In contrast thereto, it is the object of the invention to further improve a main field magnet arrangement of the above-mentioned kind such that components of permanent magnetic material with flat ground surfaces can be exclusively used for producing the main field magnet. A further object of the invention consists in further reducing the required space for a shim system disposed in the depression.
This object is achieved in accordance with the invention in a manner which is as simple as it is effective in that at least the radially inner part of the field-generating elements is formed of parallel adjacent prismatic rods of permanent-magnetic material, with the axes of the prismatic rods forming a line grid of grid constant a, wherein the surfaces of the prismatic rods of the radially inner part of the field-generating elements, facing the working volume, have different, axial separations from the center of the working volume which, in particular, are not rotationally-symmetrically distributed about the axis A extending through the center of the working volume, the line grid extending in both main directions over at least four grid constants.
It is thereby surprisingly possible to generate magnetic fields of homogeneity comparable to that of U.S. Pat. No. 5,959,454 in a working volume of comparable position and size, despite the lack of rotational symmetry and with magnet arrangements having outer dimensions of similar size. All disturbance terms of the magnetic field characterized by spherical harmonic functions with a degree l less than 5 can, in particular, be eliminated including the disturbance terms of order m=4 which are fundamentally associated with the four-fold symmetry of the inner part of the magnet arrangement. It is thereby unnecessary for e.g. individual rods to project so far into the depression as to impair accommodation of a gradient coil system and an RF coil. The field-generating elements of permanent-magnetic material advantageously generate at least 90%, preferably 99% of the homogeneous magnetic field B in the working volume.
If the main field magnet is part of a magnetic resonance (MR) tomography system, in particular a system for examining the skin, regions close to the surface, of bodies of any size can be examined using magnetic resonance methods, e.g. the skin of humans or animals. Examination of larger containers holding liquids is easy, since the specific properties of the liquid contained in the working volume is representative of all the liquid in the container.
In a preferred embodiment, the prismatic rods have the same polygonal cross-section. These rods facilitate generation of a homogeneous magnetic field and a uniform grid.
A particularly advantageous embodiment of the invention is characterized in that the prismatic rods have rectangular, preferably square cross-sections. This permits production of a particularly uniform grid. The grid constant a can be adjusted via the dimensions of the square rods. A further advantage of the design of the radially inner part of the permanent magnet arrangement of square prisms whose axes form a square line grid consists in that in this fashion, a high cross-sectional fraction vqi of the permanent-magnetic material of the radially inner part of the magnet arrangement of up to 100% is possible. This permits magnet arrangements with particularly large magnetic field strengths in the working volume and provides a particularly large amount of space in the depression. Prisms with square or rectangular cross-section can be produced in a simpler and more precise fashion than prisms having other cross-sections. The radially inner part is also particularly easy to produce. The use of prisms with square cross-section produces a radially inner part with a maximum four-fold symmetry which can principally generate disturbance terms of the order m=4. Nevertheless, such arrangements can still eliminate all disturbance terms up to and including degree l=4.
In an alternative embodiment of the invention, the prismatic rods have cross-sections of triangular shape, preferably with equal sides. This permits replacement of a square by two triangles with one axis each to improve the adjustability of the system.
In an alternative advantageous embodiment, the prismatic rods have cross-sections of a hexagonal shape, preferably with equal sides.
In an advantageous embodiment of the invention, at least groups of prismatic rods, preferably all prismatic rods, have identical construction. The square prisms of permanent-magnetic material in the radially inner part of the magnet arrangement can have the same length. This is advantageous in that the production of these prisms is particularly inexpensive owing to their uniform shape.
It is particularly advantageous if the radially outer part of the field-generating elements is also formed of parallel adjacent prismatic rods of permanent-magnetic material. Field homogenization in the working volume can be further improved with this measure.
In a preferred embodiment, the radially outer part of the field-generating elements is designed like a ring with square outer and inner ring cross-section. In this fashion, the radially outer part can be produced precisely and inexpensively by compression molding, sintering and grinding. None of the field-generating parts of the magnetic arrangement of this embodiment are rotationally-symmetric. However, a homogeneous magnetic field can still be produced in the working volume. In particular, it is possible to eliminate all disturbance terms of the magnetic field characterized by spherical harmonic functions with a degree l less than 5, even the disturbance term of the order m=4 which is fundamentally associated with the merely four-fold symmetry of the overall magnet arrangement and without e.g. individual rods projecting so far into the depression so as to impair use of the depression for accommodating a gradient coil system and an RF coil.
In a further development, the radially outer part of the field-generating elements is formed from one piece. It is possible to produce the radially outer part of this design e.g. by gluing together rods of square or cuboid cross-section.
In a preferred embodiment of the invention, the cross-sectional fraction vqi of the overall cross-sectional area of the radially inner part of the field-generating elements which is filled with the permanent magnetic material is at least 0.65. This avoids the need for individual square prisms to project deeply into the depression on the side of the magnet arrangement facing the working volume to reduce the space for receiving a gradient coil system and an RF coil in that depression.
In a preferred embodiment of the invention, the radially inner part of the field-generating elements is separated from the radially outer part. The radially inner part of the magnet arrangement therefore requires less of the expensive permanent-magnetic material with otherwise large magnetic field strength and homogeneity of the magnetic field in the working volume.
In an advantageous further development, the radial region between the radially inner part and the radially outer part of the field-generating elements is filled with a non-magnetic filler.
In a preferred embodiment of the invention, the number of prismatic rods of the radially inner part of the field-generating elements is between 37 and 81, preferably between 45 and 49. In this fashion, the number of prisms can be kept sufficiently small to make the production of the radially inner part of the magnet arrangement simple and therefore inexpensive. On the other hand, the number of prisms is large enough to obtain a particularly large spatial extent of the working volume of homogeneous magnetic field through selection of the different separations of the surfaces of the respective prisms facing the working volume from the center of the working volume. All the disturbance terms of the magnetic field characterized by a degree 1 less than 5 for the spherical harmonic functions can e.g. be eliminated even the disturbance term of the order m=4 which is fundamentally associated with the merely four-fold symmetry of the overall magnet arrangement.
In a particularly preferred embodiment of the invention, the quotient vzi of the axial separation between the surfaces, facing the working volume, of the radially inner part of the field-generating elements and the center of the working volume and the radial separation ri of the axis A extending through the center of the working volume to the next radial inner border of the radially outer part of the field-generating elements is at least 0.4. In this fashion, a depression with a relatively large amount of space is produced for receiving a gradient coil system and an RF coil and a relatively large spatial extension of the working volume with homogeneous magnetic field is possible.
In particular, this theoretically permits elimination of all disturbance terms of the magnetic field characterized by spherical harmonic functions of degree 1 less than 5, even the disturbance term of the order m=4 which is fundamentally associated with the merely four-fold symmetry of the overall magnet arrangement.
A further embodiment of the invention is characterized in that the quotient vza of the axial separation of the surfaces, facing the working volume, of the radially outer part of the field-generating elements to the center of the working volume and the radial separation ri between the axis A extending through the center of the working volume and the nearest radially inner border of the radially outer part of the field-generating elements is between 0.05 and 0.3. This guarantees that the working volume has a relatively large separation from the magnet arrangement and that the magnetic field in the working volume is sufficiently strong and homogeneous for magnetic resonance examinations. In this fashion, all disturbance terms of the magnetic field which are characterized by spherical harmonic functions of degree 1 less than 5 can be theoretically eliminated, even the disturbance term of the order m=4 which is fundamentally associated with the merely four-fold symmetry of the overall magnet arrangement.
It is particularly preferred when the quotient vra between the radial separation ra of the axis A extending through the center of the working volume to the next radially outer border of the radially outer part of the field-generating elements and the radial separation ri of the axis A to the next radial inner border of the radially outer part of the field-generating elements is between 1.5 and 2.5. This permits generation of particularly homogeneous and particularly strong magnetic fields in the working volume.
In a preferred embodiment of the invention, the quotient vl between the axial length L and the radial separation ri of the axis A extending through the center of the working volume to the next radial inner border of the radially outer part of the field-generating elements is between 1.5 and 3. This permits generation of particularly homogeneous and particularly strong magnetic fields in the working volume.
In a particularly preferred embodiment, at least part of the prismatic rods of permanent magnetic material of the radially inner part of the field-generating elements is joined to form a unit with the radially outer part and optionally with non-magnetic filler in a region between the inner and outer part. This produces a body defining axially extending channels in the region of the movable prismatic rods having e.g. square cross-sections, which can serve as guidances for the axially movable prismatic rods.
Advantageously, at least part of the prismatic rods of permanent magnetic material of the radially inner part of the field-generating elements is movable in the axial direction and the axial position of these rods can be adjusted and fixed by adjusting means. Adjustment of the axial position of several prisms permits compensation of deviations of the spatial dependence of the magnetic field in the working volume from the desired homogeneous dependence due e.g. to production tolerances.
When the adjustment means are disposed on the side of the magnet arrangement facing away from the working volume, no additional shim system is required in the depression of the magnet arrangement facing the working volume, for homogenizing the magnetic field thereby providing more space for a gradient coil system and an RF coil.
In a preferred embodiment of the invention, a magnetic field mirror, in particular a mirror plate of soft-magnetic material, is provided on the side of the magnet arrangement facing away from the working volume. While substantially maintaining the geometric shape of the magnet arrangement on its side facing the working volume, the axial length of the magnet arrangement can thereby be shortened i.e. to approximately half the length to thereby approximately halve the material costs of the relatively expensive permanent magnetic material without impairing the magnetic field strength and the homogeneity of the magnetic field in the working volume. Moreover, the undesired magnetic stray field on the side of the magnet arrangement facing away from the working volume is strongly reduced.
In an advantageous embodiment of the inventive main field magnet arrangement, the prismatic rods and optionally additional field-generating elements of the magnet arrangement are made from a permanent magnetic material having high magnetic hardness, wherein, when applying a demagnetizing magnetic field H of a strength at which induction in the magnetic material vanishes, the magnetization M(H) in the magnetized state is reduced by less than 20% relative to the saturation magnetization Mr. This assures that the permanent-magnetic material of the magnet arrangement will not demagnetize in its own magnetic field.
It is particularly preferred when the permanent-magnetic material contains a NdFeB alloy. This permits generation of particularly high magnetic field strengths in the working volume. Moreover, the inventive geometric shape of the magnet arrangement and its components can be produced precisely and inexpensively with this material.
The object in accordance with the invention is also achieved with a method for producing a magnet arrangement wherein the prismatic rods are produced by compression molding, sintering and precision grinding. In particular, prismatic rods of NdFeB can be produced with high precision and at little cost.
In an advantageous method variant, the prismatic rods of the radially outer part of the magnet arrangement are glued in parallel.
Further advantages can be extracted from the drawing and the description. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and is explained in more detail by means of embodiments.